There is no universal agreement on what a filibuster is. It can involve speaking, forcing repeated votes, or even a threat or perceived threat to block a measure.

Filibusters are not in the Constitution. They flow from the Senate’s loose rules, which impose few restrictions on debate.

The most important factor about filibusters is they take support from three-fifths of all senators to halt.

In the 100-member chamber, that means the votes of 60 senators are needed. Unless there is some bipartisan consensus, that can be a tough margin for today’s majority Democrats because they control just 54 votes, including two usually supportive independents.

Without a filibuster, approving legislation or nominations requires a mere majority.

The longest filibuster took place when Senator Strom Thurmond, then a Democrat from South Carolina, spoke for 24 hours and 18 minutes against a 1957 civil rights bill that eventually passed.

The most famous: Actor Jimmy Stewart’s filibuster in the 1939 movie, ‘‘Mr. Smith Goes to Washington.’’

Seven people nominated by President Obama to serve in federal agencies are at the center of the fight over filibuster rights in the Senate. They are:

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